Decorate addresses given to a HTTP CONNECT proxy.

This affects the Request-URI and the value of the Host header. RFC 2616
doesn't directly address the formatting of IPv6 addresses, but it
delegates some productions to RFC 2396 "Uniform Resource Identifiers
(URI): Generic Syntax," which is obsoleted by RFC 3986, which requires
square brackets for IPv6 addresses in both places.

I tested this with
	HTTPSProxy 127.0.0.1:8000
	Bridge <IPv6 bridge>
	UseBridges 1
and an Ncat HTTP proxy:
	ncat --proxy-type http -l 8000 -vvv

https://tools.ietf.org/html/rfc2616#section-3.2.1
https://tools.ietf.org/html/rfc2616#section-5.1.2
https://tools.ietf.org/html/rfc2616#section-14.23
https://tools.ietf.org/html/rfc3986#section-3.2.2